To date, it has only been possible to recover parts of the oil found in subterranean oil reservoirs because of the special adherences which retain the oil in the formation. These are a combination of capillary forces, adhesive forces, cohesive forces and hydraulic forces.
Most part of the oil will remain in the reservoir until new technology makes it possible to enhance recovery efficiency by affecting the adherences which lock the oil to the formation. Since the reserves in an oil reservoir represent vast economic resources, it is of the greatest interest to develop methods and equipment in order to be able to enhance oil recovery efficiency.
Intensive research is being carried out worldwide in the field of so-called secondary oil recovery to find methods which increase the oil yield. These include methods ranging from injection of chemicals, electrical stimulation, electromagnetic stimulation and also various methods of acoustic stimulation. Interest has been directed in particular towards the conditions surrounding the surface tension between oil and water, where attempts have been made to solve the problems involved here by injecting tensides and surfactants
In U.S. Pat. No. 5,282,508 there are described a number of different embodiments of vibrators for stimulating an oil reservoir in conjunction with electrical stimulation. These vibrators are powered by electricity, but the operation of vibrators using electric power in an oil well environment has been found to pose major practical problems with regard to insulation since the voltages used are about 1500 volts. There has thus been a need to develop a vibrator which could be operated by means of an energy source other than electricity. The most natural was to think of the use of hydraulics or gas, but the problem which arises when using a conventional vibrator as used, for example, in piling or in hydraulic hammers on excavators is that two oil supply lines must be used. In the case of short distances it is possible to use two lines, but when an oil well of several thousand meters is involved, this is impossible because of the pressure loss in the line.